Method of manufacturing a composite material

ABSTRACT

A method of manufacturing a composite material which comprises ceramic fibers in a matrix of a ceramic material. A woven mat of ceramic fibers having an electrically conductive coating thereon is immersed in a sol. The sol comprises surface-charged ceramic particles. A potential difference is applied between the fibers and a second electrode placed in the sol. The applied potential difference is continued until the mat is permeated by the ceramic particles in the sol. The mat is removed from the sol and is then heated to sinter the permeated ceramic particles.

This invention relates to a method of manufacturing a composite materialand is particularly concerned with a method of manufacturing a compositematerial using electrophoresis.

It is well known that certain characteristics of some materials can beenhanced by reinforcing those materials with a suitably configuredstructure formed from a different material. In a typical example, thestrength characteristics of one material can be enhanced by reinforcingthat material with a different suitably strong material.

In the field of ceramics it is frequently desirable to reinforce aceramic matrix material with high strength fibres of, for instance,alumina or silicon carbide. Difficulties arise, however, in ensuringthat the reinforcing fibres are completely infiltrated by the ceramicmatrix material.

One method of infiltrating reinforcing fibres with a ceramic matrixmaterial is by the use of chemical vapour infiltration. In thattechnique, the fibres are placed in an atmosphere of a suitable vapourwhich is caused to chemically break down to deposit a ceramic materialon the fibres. Ceramics such as silicon carbide can be deposited in thisway. However it is a slow process which is expensive to carry out. Inaddition, it does have a tendency to produce a matrix material which hassome degree of undesirable porosity.

Another technique is one which utilises liquid phase reaction. Thisinvolves infiltrating the reinforcing fibres with a liquid whichprogressively oxidises or reacts with a gaseous medium to form a ceramicmatrix material. For instance, the fibres could be infiltrated by moltenaluminium which is caused to oxidise to alumina as it infiltrates.

The drawback with this technique is that there is only a small range ofmaterials which are suitable for use with it. Additionally there is thedanger that unreacted metal could be left in the matrix material and thematrix material does tend to exhibit a certain degree of porosity.

A still further technique is one in which the reinforcing fibres areinfiltrated with a liquid glass precursor material which is subsequentlycrystalised to form a ceramic product.

The difficulty with this technique is that of the limited range of glassprecursor materials which are available.

It is an object of the present invention to provide a method ofmanufacturing a composite material in which such difficulties aresubstantially avoided.

According to the present invention, a method of manufacturing acomposite material comprises the steps of immersing an electricallyconductive or semi-conductive porous reinforcing medium in a ceramicsol, each of said sol particles carrying a surface charge but not havinga polymer coating thereon, applying a potential difference between saidporous reinforcing medium and a further electrode in said sol sufficientto cause said sol particles to migrate through said suspension and bedeposited upon said porous reinforcing medium, the application of saidpotential difference being continued until said porous reinforcingmedium has been substantially completely permeated by said solparticles, discontinuing said applied potential difference, taking stepsto ensure that said permeating sol particles remain in position withinsaid porous medium after the discontinuation of said potentialdifference, removing said permeated porous medium from said solsubsequently sintering said permeating sol particles within said porousreinforcing medium.

The method of the present invention is directed towards the productionof a composite material which comprises reinforcing fibres embedded in amatrix of a ceramic material.

Throughout this specification, the term "ceramic" is intended to includevitreous products as well as crystalline and semi-crystalline productsand should be construed accordingly.

The fibres are initially arranged in the particular configuration whichis desired in the final composite material. One convenient way ofachieving this is to weave the fibres in the desired configuration.However, it will be appreciated that other measures could be taken toachieve the desired fibre configuration. Indeed the fibres could bearranged in tows which are subsequently wound on to an appropriatelyshaped mandrel to produce the desired configuration.

Although the present invention is primarily intended for use withreinforcing fibres, non-fibrous reinforcement could be utilised if sodesired. Thus the present invention is generally applicable toreinforcing media which are porous. The term "porous" used throughoutthe specification should therefore be construed as embracing bothfibrous structures and other porous structures such as foamed materialsincluding foamed ceramics and reticular materials.

The fibres may be formed from any suitable electrically conductive orsemi-conductive high temperature resistant reinforcing material. Thusthey could be formed from a ceramic material such as alumina, siliconcarbide or silicon nitride coated with an electrically conductivematerial such as carbon. Alternatively they could be formed from asuitable metal.

Initially, a suspension is prepared of small ceramic particles in asuitable liquid binder, usually aqueous. The ceramic particles must besufficiently small to remain in suspension in the liquid vehicle. Wehave found therefore that it is most convenient to use a sol such as asilica sol or an alumina sol. It is important, however, that each of thesol particles should carry a surface charge.

The woven fibres, which are referred to as a fibre matt, are immersed inthe sol. A potential difference is then applied between the fibre matt,which thereby serves as one electrode, and another suitable electrodeimmersed in the sol. The polarity of the two electrodes is arranged sothat the surface charged sol particles are attracted to and thereforemigrate through the sol to be deposited upon the fibre matt byelectrophoresis.

The application of the potential difference between the electrodes iscontinued until the fibre matt has been substantially fully permeated bythe ceramic sol particles. When this has been achieved, the appliedpotential difference is discontinued.

It is important that after the applied potential difference has beendiscontinued, the deposited sol particles remain in place within thefibre matt. To this end, therefore, the particular sol chosen is onewhich is capable of gelling. Thus the sol particles gel upon depositionand thereby from a self-supporting matrix within the fibre matt.However, other means may be employed to ensure that the deposited solparticles form a self-supporting matrix. For instance, a binder such aspolyethylene oxide could be added to the sol so as to be co-depositedwith, and thereby bind together, the sol particles.

After removal from the sol, the sol particle-permeated fibre matt isdried. It is then heated at elevated temperature, preferably underpressure, in order to sinter the permeating sol particles and therebyform a ceramic matrix. The thus formed ceramic matrix is therebyreinforced by the fibre matt.

It may be possible to manufacture a particular component merely from asingle fibre matt. However this may not be possible with other, probablymore complicated components. Under these circumstances, it may bedesirable to produce a number of sol particle-permeated fibre matts andlay them up in a suitably shaped die. This, of course, has to be carriedout before the permeated matts are dry so that they are sufficientlypliable. The matts are then dried in the die and heated under pressureto sinter the sol particles.

When permeated fibre matts are laid-up in the manner described above,there can sometimes be difficulty in achieving good density and fibrevolume fraction in the resultant composite material. In order to avoidthis, it may in certain circumstances be desirable to carry out anadditional process step. After a number of permeated fibre matts havebeen produced as described above, a sol particle dip-coat is applied tothem. The dip-coat is applied from a slurry of sol particles and abinder material such as polyethylene oxide. After the fibre matts havebeen dipped in the slurry and dried several times, they are laid up inthe manner described earlier on a suitable die and heated under pressureto sinter the sol particles.

In order to demonstrate the effectiveness of the present invention, thefollowing two examples were carried out:

EXAMPLE 1

Six woven ceramic matts of carbon coated silicon carbide fibres wereprepared. The matts were prepared from a cloth which is sold under thename "Nicalon Grade 607" by the Nippon Carbon Co. Ltd. The matts wereeach 0.33 mm thick and 100 mm square.

The one matt was immersed in a silica sol which is sold under the name"Syton HT50" by Monsanto. The sol contains 50% by weight of silicaparticles and has a pH of 10.3.

The matt was connected to an electrical supply, as was a furtherelectrode which had been immersed in the sol. A potential difference of4 volts was then applied between the matt and the further electrode fora period of 10 minutes. The polarity of the applied potential differencewas arranged so that the surface charged silica particles were attractedto and deposited upon the matt. The deposited silica particles thengelled to form a matrix within the matt. Upon removal from the solvisual inspection revealed that the matt had been substantially fullypermeated by the silica particles.

The process was then repeated with the remaining five matts. Howevernone of the permeated matts were allowed to dry out.

The six wet matts were then stacked in a suitably shaped graphite die.The die was then loaded in order to compress the stacked matts.Initially a load of 0.5 tons was applied and the temperature of thematts was raised to 1400° C. at the rate of 50° C. per minute. The loadwas then increased to 10 tons and maintained at that value for 30minutes while the temperature was maintained at 1400° C. The load wasthen released and the permeated matts allowed to cool to roomtemperature.

Visual examination of the matts revealed that they had fully fusedtogether and that the silica sol particles had sintered. Subsequentmicrostructural examination revealed that there had been goodpenetration of the matts by the sol particles and there was a goodoverall surface appearance. However there was a limited amount ofmicrocracking in the interlayer region of the matrix due to silicacrystallisation.

The resultant composite material had a density of 2.16 grams/cc and afibre volume fraction of 0.59.

EXAMPLE 2

A sol was prepared by adding 2% by weight of an alumina sol to a silicasol and then milling the mixture for two hours. The silica sol was thatsold under the name "Syton 50" by Monsanto. It contains 50% by weight ofsilica particles and has a pH of 10.3. The alumina sol was that soldunder the name "Remal 20" by Remet Corporation. It contains 19.5% byweight of alumina particles and has a pH of 3.7.

The thus prepared sol was then equally divided into two portions.

Eight woven ceramic matts of carbon coated silicon carbide fibres werethen prepared. The matts were prepared from the same cloth as that usedin example 1, i.e. "Nicalon grade 607". The matts were each 0.3 mm thickand 100 mm square.

One matt was immersed in one of the sol portions and was connected to anelectrical supply, as was a further electrode which has been immersed inthe sol. A potential difference of 4 volts was then applied between thematt and the further electrode for a period of 5 minutes. The polarityof the applied potential difference was arranged so that the surfacedcharged silica and alumina particles were attracted to and depositedupon the matt. The deposited silica and alumina particles then gelled toform a matrix within the matt. Upon removal from the sol, visualinspection revealed that the matt had been substantially fully permeatedby the silica and alumina particles.

The process was then repeated with the remaining seven matts using thesame sol portion. However none of the permeated matts were allowed todry out.

The other sol portion was then taken and two aluminium plate electrodeswere immersed in

A potential difference of 32 volts was then applied between the platesfor a period of 15 minutes. This resulted in the deposition of a 4 mmthick gelled silica and alumina particle layer on one of the electrodes.

The coated electrode was then removed from the sol and allowed to dry.The dried gel coating was scraped off the electrode and then suspendedin a 10% by weight solution of polyethylene oxide binder. Each matt wasthen in turn dipped in the suspension, allowed to dry, re-dipped in thesuspension and again allowed to dry.

The matts were then stacked in a suitably shaped graphite die. The diewas then loaded in order to compress the stacked matts. Initially a loadof 6.25 tons was applied and the temperature of the matts was raised to1200° C. at the rate of 40° C. per minute. The applied load was thenincreased to 25 tons. This was held for 30 minutes whereupon thetemperature was increased to 1340° C. at a rate of 40° C. per minute.The load was then removed and the matts were allowed to cool to roomtemperature.

Visual examination of the matts revealed that they had fully fusedtogether and that the silica and alumina sol particles had sintered.Subsequent microstructural examination revealed that there had been goodpenetration of the matts by the sol particles and there was low porositywith a good overall surface appearance. The density of the resultantcomposite material was 2.26 grams/cc and it had a fibre volume of 0.25.There was no observable cracking or detectable cracking of the silicaphase.

Mechanical testing revealed that the composite material had a 3 pointbend UTS of 236.4 MPa and a Youngs Modulus of 63.54 GPa.

It may be desirable under certain circumstances to achieve a highdensity matrix. In order to achieve this a further densification step isnecessary. This can be achieved if the particular sol particles chosenare capable of remaining viscous during the compression stage, therebypermitting the use of high compression loads.

The present invention has been described with reference to themanufacture of test pieces. It will be understood however that with theuse of appropriately shaped dies, actual components could be producedfrom stacked matts subjected to suitably high compression loads.

The method of the present invention is particularly useful in themanufacture of high temperature aerospace components, ceramic tubeburners, power generation equipment, furnace components and refractorycomponents in general.

We claim:
 1. A method of manufacturing a composite material comprisingthe steps of:immersing an electrically conductive porous reinforcingmedium in a ceramic sol, said ceramic sol comprising a plurality of solparticles, each carrying a surface charge and not having a polymercoating thereon; applying a potential difference between said porousreinforcing medium and an electrode immersed in said sol, said potentialdifference being sufficient to cause said surface-charged sol particlesto migrate through said sol and be deposited upon said porousreinforcing medium, said step of applying said potential differencebeing continued until said porous reinforcing medium is substantiallycompletely permeated by said sol particles; discontinuing saidapplication of said potential difference; fixing said sol particles inposition within said porous reinforcing medium after said step ofdiscontinuing said potential difference; removing said permeated porousreinforcing medium from said sol and subsequently sintering said solparticles permeated within said porous reinforcing medium.
 2. A methodof manufacturing a composite material as claimed in claim 1 wherein saidsintering step is carried out while applying a compressive load to saidsol particle permeated porous reinforcing medium.
 3. A method ofmanufacturing a composite material as claimed in claim 2 wherein saidcompressire load is uniaxially applied.
 4. A method of manufacturing acomposite material as claimed in any one of claims 1 to 3, wherein saidporous reinforcing medium comprises fibers.
 5. A method of manufacturinga composite material as claimed in claim 4 wherein said porousreinforcing medium comprises woven fibers.
 6. A method of manufacturinga composite material as claimed in claim 4 wherein said fibers comprisea ceramic material and have an electrically conducting coating formedthereon.
 7. A method of manufacturing a composite material as claimed inclaim 6 wherein said electrically conductive coating comprises carbon.8. A method of manufacturing a composite material as claimed in claim 6wherein said fibers comprise silicon carbide.
 9. A method ofmanufacturing a composite material as claimed in claim 1 wherein saidsol particles gel upon deposition on said porous reinforcing medium tothereby maintain said permeating sol particles in position within saidporous reinforcing medium after said step of discontinuing saidpotential difference.
 10. A method of manufacturing a composite materialas claimed in claim 1 wherein said sol is a silica sol.
 11. A method ofmanufacturing a composite material as claimed in claim 1 wherein saidsol comprises sol particles of more than one material.
 12. A method ofmanufacturing a composite material as claimed in claim 11 wherein saidsol particles comprise silica and alumina particles.
 13. A method ofmanufacturing a composite material as claimed in claim 1 wherein saidsol includes a binder material which is co-deposited with said ceramicsol particles so as to bind said ceramic sol particles within saidporous reinforcing medium.
 14. A method of manufacturing a compositematerial as claimed in claim 13 wherein said binder material comprisespolyethylene oxide.
 15. A method of manufacturing a composite materialas claimed in claim 1 wherein said porous reinforcing medium comprisesat least one mat.
 16. A method of manufacturing a composite material asclaimed in claim 15 wherein said porous reinforcing medium comprises aplurality of said mats, the method further comprising a step of stackingsaid plurality of sol particle permeated mats prior to said sinteringstep.
 17. A method of manufacturing a composite material as claimed inclaim 16 further comprising a step of dipping each said sol particlepermeated mat in a suspension of sol particles and a binder, prior tosaid stacking step.
 18. A method of manufacturing a composite materialas claimed in claim 17 wherein said sol particles are the same as thosepermeating each said mat.
 19. A method of manufacturing a compositematerial as claimed in claim 17 wherein said binder comprisespolyethylene oxide.